The production of oxides by oxidation of an appropriate alkane in the presence of suitable catalyst is well known.
Brian J. Ozero, Handbook of Chemicals Production Processes, edited by Robert Meyers, McGraw Hill Book Co. (1986) at Chapter 1.5, discusses cyclic processes using both oxygen and air as oxidant for the production of ethylene oxide (EO) from ethylene. In these processes, the alkene is oxidized in a multitubular catalytic reactor in vapor phase. The reactor off gases are cooled and scrubbed with water in an absorber to recover ethylene oxide which is sent to the recovery section for further purification.
In the oxygen-based process described by Ozero, the scrubber off gases are divided into three parts which are: i) recycled to the reactor, ii) vented and iii) sent to a separator for carbon dioxide removal and recycle of the remaining hydrocarbons. This process suffers from several disadvantages. For example, the oxygen-based ethylene oxide process requires a separate carbon dioxide removal unit and a purge to remove argon to prevent its accumulation.
In the air-based process described by Ozero, the scrubber off gases are sent to a second reactor, which is the purge reactor, where additional unreacted ethylene is reacted using a higher air to ethylene ratio, foregoing some EO selectivity. The reactor off gases are again passed through another water scrubber to recover EO produced. It is known that the volume of hydrocarbons purged, when utilizing air as a source of oxygen, requires that the purge scrubber off gases be incinerated to remove any remaining hydrocarbons in order to meet environmental regulations. In this air-based process, an additional purge oxidation reactor, a water scrubber, and an effluent incinerator are required, as well as a greater volume of catalyst. Also, both this and the oxygen-based process use the expensive ethylene as the raw material. The processes described by Ozero are limited to either pure oxygen or air.
More recently, Khoobiar et al., in U.S. Pat. No. 4,609,502 disclosed a cyclic process for producing acrylonitrile using propane as a starting material which is initially dehydrogenated catalytically in the presence of steam to form propylene. This is in contrast to most conventional dehydrogenation processes which avoid steam primarily due to the costs involved. After ammoxidation, the effluent is quenched, the desired product removed, and the off-gases, including propylene and propane, are sent to an oxidation reactor to remove oxygen by selective reaction with hydrogen to form water vapor. The gas mixture exiting the selective oxidation reactor includes substantial amounts of methane, ethane and ethylene, which are by-products of dehydrogenation, and unreacted propylene in addition to carbon oxides. As an option, this gas mixture is split and a portion is sent to a separator which removes only carbon dioxide. A portion of the effluent from the separator is purged to remove light hydrocarbons. The nonpurged stream is mixed with the remainder of the oxidator effluent, fresh propane and steam, if necessary, and sent to the dehydrogenator where the propane is converted to propylene. Another option is to cool and liquify the C.sub.3 hydrocarbons therefrom and then vaporize them prior to recycle.
The aforementioned process suffers from several disadvantages. For example, there is no practical way to selectively remove by-products of propane dehydrogenation, such as methane, ethane, ethylene and the like, thereby preventing their accumulation in the system other than by removing them in the purge stream. The removal of these gases in a purge stream will result in a loss of some of the circulating propane and propylene. As the process is being carried on in a continuous manner, this loss of starting material causes a significant decrease in the yield of propylene. As mentioned above, propane and propylene can be recovered from the purge stream prior to venting. This requires additional refrigeration apparatus to cool and liquify propylene and propane. The separated C.sub.3 hydrocarbons must be vaporized prior to recycle. These operations add to the capital costs and power requirements of the process.
Another disadvantage of the Khoobiar et al. process stems from the use of the selective oxidation reactor to treat the gaseous effluent from the quencher. The gases exiting the quencher are at ambient temperature and must be heated prior to introduction into the oxidation reactor in order to promote oxygen removal. Because there is a significant amount of oxygen in the quench effluent, the heat of reaction generated in the oxidation reactor can result in excessive temperatures in the system. There are three options to alleviate this problem. First, the amount of oxygen entering the oxidation reactor can be reduced by other means. Second, multiple reactors can be utilized with a cooling means between each pair of reactors. Third, a portion of the effluent from the reactor is passed through a cooling means and recycled to the feed to reduce the internal temperature of the reactor. None of these measures is attractive from the viewpoint of cost and efficiency.
The oxidation reactor in the Khoobiar et al. process is operated with oxidation catalysts such as noble metals (e.g., platinum). Olefins and carbon monoxide, which are generated in the dehydrogenation reactor, are known to deactivate these catalysts, as disclosed in Catalytic Processes and Proven Catalysts, Charles L. Thomas, Academic Press (1970) pp. 118-119. Accordingly, multiple oxidation reactors must be used to allow for frequent regeneration of the catalyst which represents yet another addition to production costs (U.S. Pat. No. 4,609,502, column 4, lines 51-56).
It is therefore apparent that the industry is still searching for a cost effective process of converting hydrocarbons into oxides. Applicants have discovered a process which is cost effective and in which the disadvantages of the aforementioned systems are substantially reduced or eliminated. Moreover, in comparison to conventional processes, the thermal requirements of Applicants'process are markedly reduced.